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Temperature scales

The most commonly used measurement of temperature is the Celsius scale. The units of this scale are degrees Celsius (°C). This scale was designed to reference the freezing point (0°C) and the boiling point (100°C) of water.

There is no upper limit, but the lowest temperature possible is -273°C. At this temperature, almost all matter is solid and the vibrations of particles are incredibly small. Absolute zero is the coldest temperature possible. At absolute zero, all the particles in a substance stop moving. They have no energy left to lose, so the substance cannot get any colder.

Many scientists use the Kelvin scale instead. Kelvin is a better scale to use in space where there is no water.

Kelvin begins at absolute zero and so there are no minus numbers in the Kelvin scale, which makes calculations simpler. The units are kelvin (K). An increase in one degree Celsius is the same as one kelvin.

Conversion between the scales:

temperature in degrees Celsius (°C) = temperature in kelvin - 273

temperature in kelvin = temperature in degrees Celsius (°C) + 273

To be able to measure temperature easily we require fixed points. Two common fixed points are the melting and boiling points of water. These are 0°C and 100°C respectively.

Thermometers measure temperature. The liquid (usually mercury or coloured alcohol) expands when heated which means that it rises up the glass to show a higher temperature.

Close-up of a thermocouple component

A thermocouple is used to measure very high temperatures or those that change quickly. Thermocouples contain two different metals. A voltage is produced where these metals meet inside the thermocouple. This voltage is directly related to temperature.

Thermal capacity

The thermal capacity of an object is the amount of heat required to change the temperature of the object by a certain amount. This is measured in joules per kelvin (J/K). The thermal capacity of a block of copper can be determined by heating it and dividing the heat energy used by its temperature change.

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Glossary
1. force A push or a pull. The unit of force is the newton, 'N'.
2. inversely proportional A relationship between two variables where as one variable increases, the other variable decreases. Eg as the volume doubled, the pressure decreased by half.
3. Kelvin scale Scale of temperature directly relating temperature to pressure.
4. kinetic energy The energy that moving objects have.
5. Pa Pascals, the unit of pressure.
6. pressure Force exerted over an area. The greater the pressure, the greater the force exerted over the same area.
7. proportional When two quantities have the same ratio or relative size. For example, current is proportional to voltage if the current doubles when the voltage is doubled.
8. thermocouple A measuring device made from two different metal wires joined together in two places or 'junctions'. If one junction is hotter than the other, a potential difference forms which an be measured.
9. voltage The potential difference of a cell, electrical supply or electric component. It is measured in volts, 'V'.